Hayama, Shusaku; (2001) Neutron and X-ray diffraction studies of metal amine solutions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The structure of solutions of lithium in ammonia and methylamine has been studied over the concentration range 2 to 22 MPM using two complementary techniques; neutron and X-ray diffraction. It was found that, in spite of differences in the electrical and physical properties, the structural changes induced by the introduction of lithium metal into ammonia and methylamine are similar, and that both solutions are highly structured over short- and intermediate-length scales. The latter finding is consistent with Mott's suggestion that the metal-nonmetal transition is dominated by electron-electron interactions rather than disorder (Anderson-type). The first detailed studies of the ion-solvent and solvent structure in lithium-methylamine solutions by the method of isotopic labelling (natLi/6Li and D/1H) in neutron diffraction are reported. The results show that each Li+ cation is strongly solvated by about four solvent molecules, with the nearest-neighbour Li-N distance at 2.15(2) and 2.09(2) A for 18 and 14.5 MPM respectively. This confirms that the excess electrons reside outside the first solvent shell of Li+, and electrical conduction is by Brownian-like hopping of electrons between neighbouring localised sites formed by the solvated cations and free solvent molecules. The solvent-solvent structure shows the presence of relatively strong hydrogen-bonding in methylamine at 230 K: approximately one hydrogen bond per molecule, nNH = 1.0(3), at rNH = 2.3(1) A. As the concentration of metal is increased, hydrogen-bonding and the coordination number of nearest neighbour are progressively perturbed. Qualitatively similar trends have also been observed by X-ray diffraction studies of lithium-ammonia solutions. These trends are consistent with the Bjerrum-type defect model of a polaronic cavity (Catterall and Mott 1969), which suggests that the overall decrease in density is due to ejection of solvent molecules from around excess electrons.

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